Asters produce bright daisy-like flowers from late summer through autumn. Some varieties of this popular perennial plant grow to 8 inches (20 cm) while others grow as tall as 8 feet (2.4 m), but the growing requirements for all varieties are very similar.

Steps

Part 1

Sowing Seeds Indoors

1

Prepare the seeds in winter. If you choose to sow seeds indoors, you should do so roughly one to two months before your expected transplant date.

Note that seed germination tends to be fairly uneven, so do not expect all of the seeds you sow to grow.

Since seed germination is so unpredictable, many gardeners prefer to buy seedlings from a garden nursery or to use plants divided from previously established asters.

2

Fill small containers with seed starting medium. Fill the compartments of a large plastic seedling tray or flat with a seed starting soil blend.

You can use plastic cups, pots, or other small containers if you do not have a seedling tray. The containers should be 3 to 4 inches (7.6 to 10 cm) deep.

3

Sow the seeds. Place one seed in each seedling compartment. Push the seed into the soil until it is about 1 inch (2.5 cm) deep.[1]

Lightly brush soil over the hole created by the seed after setting it into a compartment.

4

Store in the refrigerator. Loosely cover the seedling tray with plastic wrap and place the entire thing in a refrigerator. Keep it there for four to six weeks.

Chilling the seeds artificially mimics the chilling process seeds would undergo in nature. Using a refrigerator instead of using the cold ground outdoors ensures that the seeds will not freeze and die.

5

Transfer to a sunny spot. Remove the seeds from the refrigerator roughly two to four weeks before the last expected frost. Place the tray in a sunny spot indoors.

This spot should receive at least six hours of sunlight each day.

You will need to wait until the seedlings form before you can move anything outdoors. This will usually happen quickly.

Part 2

Transplanting Seedlings Outdoors

1

Wait until spring. Transplant aster seedlings outdoors in early to mid spring after the threat of frost has passed.

This is true regardless of whether you are working with seedlings started indoors, seedlings bought from a nursery, or plants divided from previously established asters.

2

Select a sunny spot with well-drained soil. Asters thrive in sites receiving full sun to partial shade. The soil can be rich or of average quality, but it must be capable of draining well.

Avoid planting asters in heavy clay soils since these tend to drain poorly.

Planting asters at the top of a slight incline or hill can improve soil drainage, but it is not strictly necessary to do so.

3

Amend the soil. Unless the soil is notably rich already, you should mix a little nutrient dense compost into it before transplanting the asters.

Use a garden fork or tiller to loosen the top 12 to 15 inches (30 to 38 cm) of soil at the planting site.

Add 2 to 4 inches (5 to 10 cm) of compost. Mix this compost into the loosened soil using the garden fork.[2]

4

Dig deep holes for each aster plant. Each hole should be twice as wide as the diameter of the seedling compartment or pot currently holding the aster plant. The depth of the hole should be roughly the same depth as the current container.

Carefully remove the seedling. Gently press on the sides of the plastic compartment holding each seedling. Start from the bottom and slowly work your way up. The seedling, its root ball, and the attached soil should ease out of the compartment.

If you have difficulty removing the seedling, dampen the soil with water first. Wet soil is more compact and easier to move.

If you cannot press on the sides of the container to remove the seedling, tip the container on its side and carefully insert a trowel down one side. Wiggle the trowel around inside the pot until you can glide it out along with the root ball and attached soil.

6

Place the seedling in a planting hole. Place each aster plant in the center of its planting hole so that the top of the root ball is even with the surface of the soil around it.

Carefully fill in the rest of the hole around the root ball with some of the soil you previously removed from the planting site.

Use your hands to gently pat the soil firmly in place.

7

Water well. As soon as the seedlings are in the ground, you should water the soil thoroughly to help settle the soil and encourage the plants to establish themselves.

There should not be any major puddles on the surface of the soil, but the soil should be visibly damp.

Part 3

Caring for Established Asters

1

Cover the area with mulch. Surround the asters with 2 inches (5 cm) of mulch immediately after planting and every spring.

Before adding new mulch in the spring, remove any old mulch.

Mulch keeps the soil cool in the summer and warm in the winter. It also helps to limit and prevent weed growth.

2

Water as needed. Monitor the amount of rain you get each week during the growing season. If you get less than 1 inch (2.5 cm) of rain during one week, you should soak the soil of the planting site.

Asters are sensitive to moisture and will usually become visibly weakened if they receive too much or too little moisture.

Plants that receive too little water will usually lose flowers and leaves.

Plants that receive too much water may start to yellow and wither.

3

Enrich the soil with an appropriate fertilizer. At minimum, you should mix a thin layer of compost into the soil each spring before new growth starts to form.

For even better results, mix a balanced, general purpose fertilizer into the soil once a month. Apply the fertilizer as directed on the package label.

4

Prune twice a year. You'll need to do some light pruning in the spring and some heavy pruning in the fall.

Pinch young shoots in the spring to redirect growth outward. Doing so will create a bushier plant.

Cut the entire aster plant back once the foliage dies in the winter. Cut down stem portions that appear sickly or scraggly, or trim the stems back completely to 1 or 2 inches (2.5 or 5 cm) above the soil line. Most aster varieties can withstand either option. Cutting the plant down can improve its long-term growth habit, but doing so will also delay flowering by several weeks.

If you live in a colder climate (USDA hardiness zones 5 and below), you may want to wait until spring before doing major pruning. Leaving the plants intact over harsh winters can improve their odds of survival.[3]

You can also remove dead flower blooms on an ongoing basis to improve the overall appearance of the plant, but doing so is not necessary for the health of the plant.[4] If you do deadhead old blooms, do so carefully since new buds tend to be located nearby.

5

Stake tall varieties. Many asters can grow without staking, but if you have a larger variety that begins drooping, install a stake and train the foliage upright.

The stake you choose should be about 12 inches (30 cm) taller than the current height of the plant.

Hammer the stake into the ground roughly 2 to 3 inches (5 to 7.6 cm) away from the main stem of the plant.

Use wool yarn or nylon stockings to gently tie the branches of the plant along the height of the stake.

6

Divide the plants every two to four years. Dividing the plant as it gradually becomes fuller will allow it to distribute its resources more effectively. As a result, the plant will remain vigorous and the flowers will remain plentiful.

Wait until spring before dividing plants.

Carefully dig out half to two thirds of the established plant. Leave the remainder in its current location.[5]

Divide the portion you dug out into two or more sections. Each clump you divide off should have three to five shoots.

These divided portions can be planted in another area of your garden or a friend's garden. Treat these divided portions as new seedlings and transplant them accordingly.

7

Watch out for pests and disease. Asters don't often have difficulties with pests and diseases, but some varieties can fall victim to powdery mildew, rusts, white smut, leaf spots, stem cankers, aphids, tarsfonemid mites, slugs, nails, and nematodes.

Prevention is better than treatment. Your best option is to choose disease-resistant aster varieties for your garden.

When problems do arise, treat them with an appropriate pesticide or fungicide.